I don't think there is a storage problem, and once again, pruned nodes would be fine for most. I wonder how many people are actually running full nodes right now. The speed is dictated by your internet. I imagine most people are not hosting on their own infrastructure either, but rather somewhere in the cloud, which will be highly accessible in terms of speed. 

 Good point  @bitbybit. Decentralization is the limiter of speed. But even Litecoin has 2.5 minute block time. Not any more or less usable than BTC. But it is faster.

To me, usable has to be 4 sec or less. I'm testing and learning Nano now. It's always been of interest to me. Checks all the boxes, fixed supply (all mined up front), instant transactions. 

I don't understand its mining/consensus yet. Maybe a dud. We'll see.

I think Bitcoin has a wakeup call. It can remain as "digital gold", slow, store of value. But that means people use something else for daily transactions. 

Lightning, or another L1. To me, the usability difference between Lightning vs another good L1 is minimal. Risks, exchange time, costs. In fact Lightning probably fares worse at these. 

So if a faster L1 protocol comes out with moderate security, capital will flow there for payments. But it may start accumulating there and avoiding going back to Bitcoin at all if/when that L1 protocol proves itself. Very likely scenario IMO.  

 I don't think any other crypto has a chance at success.  Btc either works or we should all be owning gold/silver